Books, magazines and other publications are typically produced by offset or gravure printing. In offset printing, a plate is imaged in a contact frame or by direct exposure to a laser beam. The plate is later mounted on a press and then impressed on the printed substrate. In gravure printing, a cylinder is engraved by laser exposure and subsequent chemical etching or by the use of a penetrating pyramid shaped diamond stylus in an apparatus known as a "helioklischograph".
The formation of the printing plate for offset printing or the cylinder for gravure printing, is error prone and very often defects occur. Such errors may arise from a number of sources. In offset printing the plates are routinely imaged in vacuum contact frames or Step and Repeat machines such as Impomaster.TM. (manufactured by Misomex AG, Hagerstrom, Sweden). In both cases the plate is placed on a surface in contact with a color separation film. A vacuum is applied to ensure uniform attachment and the absence of a gap between the film and the plate. If air pockets (or dust particles) remain between the film and the plate, the exposed image will have non-uniform areas. In addition, errors in plate exposure may be caused by scratches, dust and paper/cloth lint that remain on the film after cleaning.
After inspection of the plates, which was hitherto mainly visually performed, or performed with the aid of an inspection station such as that available from Just Normalicht, Newton, Pa., USA, scratches and pinholes on non-imaged areas may be deleted by a correcting pencil. However, errors in imaged areas and errors related to separation or imposition order cannot be corrected at all.
Gravure cylinders are typically engraved by the use of a pyramid shaped diamond stylus which is forced into the copper cylinder forming gravure cells. One source of defects in the manufacturing process of gravure cylinders is faults which occasionally occur in the stylus, e.g. a chopped tip, stylus rib defects and others which may be caused, for example, by stylus material fatigue, rib wearing, etc. In addition, abnormal gravure cells are formed at times by inaccuracies in the current which drives the stylus. An additional source of engraving errors is regional copper re-crystallization on bare cylinders, particularly where the cylinders are stored for long periods of time (i.e., 2-3 weeks). The re-crystallization areas respond differently to stylus pressure than those which have not re-crystallized, and the cell and its walls receive a distorted form.
By virtue of such defects, plate or cylinder waste is typically between 2-10% depending on the print's desired quality and in view of the high costs of gravure cylinders, this fact poses a serious financial problem in this industry.
One technique of quality control of gravure cylinders involves the formation of three parallel reference test lines with depth/size corresponding to 5, 50 and 100 dot percentage prior to the engraving of gravure cells and also at the end of the engraving process. The two sets of lines are compared and the extent of discrepancy, if any, is proportional to the error extent in the process of formation of the gravure cells. The method has an inherent shortcoming in that the cylinder's quality can only be evaluated post factum at a stage in which nothing can be done anymore to correct the defects in the already formed cells. Accordingly, in case the variation exceeds a certain allowed threshold, depending on the exact quality of the printed job, the cylinder should be discarded and a new one has to be engraved.
By an alternative approach, the size of gravure cells is measured manually by auxiliary means such as, for example, Dotcheck.TM. (Twentse Graveer Industrie B.V., Entschede, The Netherlands). However, such cell measuring systems operate off-line and cannot be applied during the engraving process. Additionally, such a manual process depends on personal skills, and as such, is susceptible to inaccuracies depending on the individual who conducts the inspection.
It is the object of the present invention to provide an automated printing plate and printing cylinder inspection method and system, capable of inspecting offset plate and gravure cylinders and detecting defects of the kind specified during or after the manufacturing process thereof.
It is a further object of the present invention to provide such method and system in which the inspection is performed in a reliable and reproducible fashion, which does not interfere in the manufacturing process, and which is essentially independent on human skills.